Partially degradable fibers and microvascular materials formed from the fibers

ABSTRACT

A partially degradable polymeric fiber includes a thermally degradable polymeric core and a coating surrounding at least a portion of the core. The thermally degradable polymeric core includes a polymeric matrix including a poly(hydroxy-alkanoate), and a metal selected from the group consisting of an alkali earth metal and a transition metal, in the core polymeric matrix. The concentration of the metal in the polymeric matrix is at least 0.1 wt %. The partially degradable polymeric fiber may be used to form a microvascular system containing one or more microfluidic channels.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/040,287, filed Sep. 27, 2013, which claims the benefit ofU.S. Provisional Application No. 61/708,149 entitled “Partially DegradedFibers And Microvascular Materials Formed From The Fibers” filed Oct. 1,2012, which applications are incorporated herein by reference in theirentirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under contract numbersFA9550-10-1-0255, awarded by the Air Force Office of ScientificResearch, and DOE ANL 9F-31921, awarded by the Energy Frontier ResearchCenters (EFRC). The government has certain rights in the invention.

BACKGROUND

Synthetic composite materials possess desirably high strength-to-weightratios; however, synthetic composites typically have lacked dynamicfunctionality that occurs in natural composite materials. Naturalcomposite materials can utilize vascular networks to accomplish avariety of biological functions, in both soft and hard tissue. Forexample, composite structures such as bone tissue or wood arelightweight and have high strength, yet contain extensive vasculaturecapable of transporting mass and energy.

Another feature of natural composite materials is their ability to havedifferent levels of communication between distinct vascular networks,depending on the specific role of the material within an organism. Inone example, the phloem and xylem channels of plant vascular bundles maybe independent and separated by lignin layers, such that the contents ofthe channels do not interact. In another example, physical contactbetween the artery and vein channels of a human or animal circulatorysystem may provide for heat transfer between the channels; however, thechemical compositions of the fluids in these channels remain separate.In yet another example, gas exchange of oxygen and carbon dioxide occursbetween the blood vessel channels and the alveoli channels within thelungs.

An ongoing challenge in materials science is the development ofmicrovascular networks in synthetic materials using conventionalmanufacturing processes. Specialized fabrication methods such aslaser-micromachining, soft lithography, templating with degradable sugarfibers, and incorporating hollow glass or polymeric fibers can producesome microvascular structures in synthetic materials. These specializedmethods, however, are not currently suitable for rapid, large-scaleproduction of materials having complex vasculatures.

In one approach to microfluidic materials, relatively short microfluidicchannels are provided in a matrix in the form of hollow glass fibers (WO2007/005657 to Dry). The glass fibers are present as repair conduitscontaining a fluid that can heal a crack in the composite matrix. Asignificant limitation of this approach is the brittle nature of thehollow glass fibers, which limits the shapes and lengths of microfluidicchannels that can be present in the material. In addition, the glassfibers cannot readily be used to form a microfluidic network.

In another approach to microfluidic materials, microfluidic channels areformed in a polymeric matrix by arranging hollow polymeric fibers andthen forming the matrix around the hollow polymeric fibers (U.S.Publication No. 2008/0003433 to Mikami). Hollow polymeric fibers mayoffer a wider variety of microfluidic channel shapes than thoseavailable from hollow glass fibers. This approach, however, also has anumber of limitations, including an inability to form a network from theindividual hollow fibers, the relatively small number of materialsavailable as hollow fibers, and the possibility of incompatibilitybetween the hollow fiber and the matrix.

Microfluidic networks can be formed in a polymeric matrix using athree-dimensional (3-D) direct-write assembly technique (U.S.Publication No. 2008/0305343 to Toohey et al.). While this fabricationmethod provides excellent spatial control, the resulting networkstypically will not survive the mechanical and/or thermal stressesencountered in the conventional processes of forming reinforcedcomposites.

It is desirable to provide multiple microvascular networks in syntheticmaterials, where the type and level of communication between thedistinct microvascular networks can be varied between differentmaterials. It is desirable for such complex microvascular networks to beformed using conventional manufacturing processes. It also is desirablefor the microfluidic channels of the different networks within amaterial to be available in a variety of shapes and dimensions, and fora variety of polymers to be available as the polymeric matrix of suchcomposites.

SUMMARY

In one aspect, there is provided a partially degradable polymeric fiberthat includes a thermally degradable polymeric core and a coatingsurrounding at least a portion of the core. The thermally degradablepolymeric core includes a polymeric matrix including apoly(hydroxyalkanoate), and a metal selected from the group consistingof an alkali earth metal and a transition metal, in the core polymericmatrix. The concentration of the metal in the polymeric matrix is atleast 0.1 wt %.

In another aspect, there is provided a method of making a partiallydegradable polymeric fiber that includes combining a fiber containing apoly(hydroxyalkanoate) and having an exterior surface, and a compositioncontaining a fluorinated fluid and a metal selected from the groupconsisting of an alkali earth metal and a transition metal. The methodfurther includes maintaining the fiber and the composition together at atemperature and for a time sufficient to provide a concentration of themetal in the fiber of at least 0.1 wt %, separating the fiber and thefluorinated fluid, and depositing a layer of a material on at least aportion of the exterior surface of the fiber.

In another aspect, there is provided a partially degradable fiber systemthat includes a thermally degradable polymeric core and a coatingsurrounding at least a portion of the core. The thermally degradablepolymeric core has a degradation temperature of at most 250° C. At leasta portion of the coating does not thermally degrade at temperaturesbelow 275° C.

In another aspect, there is provided a microvascular system thatincludes a solid polymeric matrix including a first material, and awoven structure in the matrix. The woven structure includes a pluralityof microfluidic channels having a channel wall including a secondmaterial, where the second material is different from the firstmaterial.

In another aspect, there is provided a method of making a microvascularsystem that includes forming a composite including a solid polymericmatrix, and a plurality of partially degradable polymeric fibers in thematrix. The partially degradable polymeric fibers include a thermallydegradable polymeric core having a degradation temperature of at most250° C., and a coating surrounding at least a portion of the thermallydegradable polymeric core, where at least a portion of the coating doesnot thermally degrade at temperatures below 275° C. The method furtherincludes heating the composite to a temperature of from 100 to 250° C.,maintaining the composite at a temperature of from 100 to 250° C. for atime sufficient to form degradants from the polymeric core, and removingthe degradants from the composite to provide a network of microfluidicchannels. The degradants have an average molecular weight less than 500Daltons.

In a further aspect, there is provided a method of making a microporousfilm that includes providing a coating precursor including a thermallydegradable polyhydroxyalkanoate having a degradation temperature of atmost 250° C., and a thermally stable material that does not thermallydegrade at temperatures below 275® C. The method further includescasting a film of the coating precursor, solidifying the coatingprecursor, and heating the film to a temperature of from 100° C. to 250°C. for a time sufficient to form degradants from the thermallydegradable polyhydroxyalkanoate. The method also includes removing thedegradants from the film to provide a microporous film. The microporousfilm may serve as separator in energy storage devices such aslithium-ion batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter of the present application can be better understoodwith reference to the following drawings and description. The componentsin the figures are not necessarily to scale and are not intended toaccurately represent molecules or their interactions, emphasis insteadbeing placed upon illustrating the principles of the subject matter ofthe present application. Moreover, in the figures, like referencednumerals designate corresponding parts throughout the different views.

FIG. 1 depicts a partially degradable polymeric fiber.

FIG. 2 is a schematic representation of a method of making a partiallydegradable fiber.

FIG. 3 is a schematic representation of a method of making a partiallydegradable fiber.

FIGS. 4A and 4B depict examples of an apparatus for depositing a coatingon a degradable fiber.

FIG. 5 depicts a composite material containing a partially degradablepolymeric fiber.

FIG. 6 depicts a microvascular system.

FIG. 7 is a schematic representation of a method of making amicrovascular system.

FIG. 8 illustrates the formation of a microvascular system from acomposite material containing a partially degradable polymeric fiber.

FIG. 9 illustrates the formation of a microvascular system having twomicrochannels, from a composite material containing two partiallydegradable polymeric fibers.

FIG. 10 is a schematic representation of a method of making amicrovascular system.

FIG. 11 depicts a microvascular system having independent microchannels.

FIG. 12 depicts a microvascular system having microchannels in thermalcommunication.

FIG. 13 depicts a microvascular system having microchannels in fluidcommunication.

FIG. 14 represents a graph of the diameter of a coated fiber as afunction of the distance along the fiber.

FIG. 15 depicts a scanning electron microscopy (SEM) image of acomposite material containing a partially degradable polymeric fiber.

FIG. 16 depicts a micro-CT image of two independent channels formed fromtwo partially degradable polymeric fibers.

FIG. 17 depicts images of a composite material containing twoindependent channels formed from two partially degradable polymericfibers.

FIG. 18 depicts a SEM image of a porous polyester material.

FIG. 19 depicts SEM images of a film containing a blend of polyimide andpoly(lactic acid) (PLA).

FIG. 20 depicts SEM images of the film according to FIG. 19 afterthermal degradation and removal of the PLA.

FIG. 21 depicts SEM images of a film formed from a bicontinuous filmcontaining polyimide and PLA, after thermal degradation and removal ofthe PLA.

FIG. 22 is a schematic representation of a method of making amicroporous film.

FIG. 23 depicts linear sweep voltammograms (LSV) of lithium-ion coincells assembled with a microporous polyimide separator or a CELGARD®2325 separator.

FIG. 24A represents galvanostatic plots for coin cells assembled with amicroporous polyimide separator (40:60=PI:PLA, 10 mins annealing) orwith a CELGARD® 2325 separator as control. Only one cycle is shown. FIG.24B represents a graph of the discharge capacities of coin cellsassembled with microporous polyimide separators with different PI:PLAratios (10 mins annealing time) or a CELGARD® 2325 separator as control.Data is shown for 10 cycles. FIG. 24C represents a graph of dischargecapacities for coin cells assembled with microporous polyimideseparators (40:60=PI:PLA, 10 mins annealing) or with a CELGARD® 2325separator as control. The discharge capacities were measured at C/10,C/5, C/2, and C/1 rates, respectively.

FIG. 25A represents a graph of thermal shrinkage measured at increasingtemperatures for a microporous polyimide separator and a CELGARD® 2325separator. FIG. 25B depicts a photograph of a microporous polyimideseparator and a CELGARD® 2325 separator before and after heat treatmentat 140° C. for 30 minutes.

FIG. 26 depicts an optical micrograph of the cross-section of a PLAfiber with a PI/PLA coating.

FIG. 27 depicts an optical micrograph of the cross-section of a set ofPLA fibers held together with a PI/PLA coating.

DETAILED DESCRIPTION

In one aspect, a partially degradable polymeric fiber includes athermally degradable polymeric core and a coating surrounding at least aportion of the core. The partially degradable polymeric fiber may beused to form a microvascular system having a solid polymeric matrix anda woven structure in the matrix, where the woven structure includes aplurality of microfluidic channels having a channel wall, which may haveany of a variety of permeability properties.

Such microvascular systems can provide unprecedented applications, andcan be designed to contain a variety of microvascular network types andsizes—from simple, straight conduits to complex, computer-controlled 3Dwoven architectures. The microvascular systems may be formed fromcommercially available materials, and may be integrated seamlessly withconventional fiber-reinforced composite manufacturing methods.

A microvascular system may be formed from composite materials containingpartially degradable polymeric fibers and optionally containingreinforcing fibers. Partially degradable polymeric fibers may be used toform biomimetic material systems in a reliable manner, and may be usedto model, reproduce and/or extend transport functions performed bymicrovascular systems in nature. Composite materials containing bothpartially degradable polymeric fibers and reinforcing fibers can be usedto provide microvascular systems, such as those described above.

A partially degradable polymeric fiber may include a thermallydegradable polymeric core that degrades at temperatures above thosetypically used for forming composite materials, but below the typicaldegradation temperatures of composite materials. The thermallydegradable polymeric core may be at least partially surrounded by acoating, where at least a portion of the coating does not thermallydegrade at the same temperatures as the core. The thermally degradablepolymeric core may include a polymeric fiber matrix and a catalyst inthe fiber matrix that lowers the degradation temperature of the matrixpolymer to within an appropriate temperature window.

Partially Degradable Polymeric Fibers

FIG. 1 depicts a schematic representation of a partially degradablepolymeric fiber 100, which includes a thermally degradable polymericcore 110 and a coating 120 surrounding at least a portion of thethermally degradable polymeric core. The thermally degradable polymericcore 110 includes a polymeric matrix. Preferably the polymeric matrix ofthe core 110 has a degradation temperature of at most 250° C., whereasat least a portion of the coating 120 preferably does not thermallydegrade at temperatures below 275® C.

Partially degradable polymeric fibers, such as fiber 100, preferablyhave a combination of desirable properties. These desirable propertiesinclude sufficient strength for configuration as a preform and/or forcombination with a composite matrix precursor using standard compositeformation methods, and mechanical integrity at temperatures typicallyused to form composites. The desirable properties also includedegradation and volatilization temperatures within a range between thehighest composite matrix solidification temperatures and the lowestthermal degradation temperatures of the composite matrix. Preferably thepartially degradable polymeric fiber has all of these desirableproperties.

The thermally degradable polymeric core 110 preferably has a degradationtemperature below 280° C., and preferably has a degradation temperatureof at most 250° C. Preferably the thermally degradable polymeric core110 has a degradation temperature between 100 and 250° C. Preferably thethermally degradable polymeric core 110 has a degradation temperature ofat most 220° C., of at most 180° C., of at most 150° C., or of at most100° C.

The thermally degradable polymeric core 110 may include apoly(hydroxyalkanoate). A poly(hydroxyalkanoate) is an aliphaticpolyester having the general structure:

O—C(R¹R²)—(CR³R⁴)_(x)—C(═O)

_(n)where n is an integer of at least 10, x is an integer from 0 to 4, andR¹-R⁴ independently are —H or an alkyl group. Examples ofpoly(hydroxyalkanoate)s include poly(3-hydroxybutyrate) (P3HB),poly(4-hydroxybutyrate) (P4HB), poly(3-hydroxy-valerate) (PHV),polycaprolactone, poly(lactic acid) (PLA), poly(glycolic acid) (PGA),and copolymers of the monomeric units of these polymers.

It has now been discovered that thermally degradablepoly(hydroxy-alkanoate) fibers can be at least partially surrounded witha coating to produce partially degradable fibers, which can be usedsuccessfully as coated sacrificial fibers in polymeric matrices such asepoxies. These coated sacrificial fibers preferably include a thermallydegradable fiber core having a polymeric fiber matrix including apoly(hydroxyalkanoate) and including a metal in the fiber matrix, wherethe metal includes an alkali earth metal and/or a transition metal.Preferably the concentration of the metal in the fiber matrix is atleast 0.1 percent by weight (wt %).

Poly(hydroxyalkanoate)s may degrade at elevated temperatures through adepolymerization process, forming small molecule degradants that may begases. For example, poly(lactic acid) (PLA) is a thermoplasticpoly(hydroxyalkanoate) that depolymerizes at temperatures above 280° C.,forming lactide as a gaseous degradant. The depolymerization temperatureof poly(hydroxyalkanoate)s such as PLA may be reduced by blending thepoly(hydroxyalkanoate) with an alkaline earth metal and/or a transitionmetal. A reduced depolymerization temperature may help prevent damage tomaterials in contact with the thermally degradable polymeric core 110,such as the coating 120 or a polymeric matrix in which the partiallydegradable polymeric fiber 100 is contained. Preferably the thermallydegradable polymeric core 110 depolymerizes within an appropriatetemperature range, but without deterioration of the desirable mechanicalproperties of the fiber 100 below the degradation temperature.Preferably the poly(hydroxyalkanoate) has a degradation temperaturebelow 280° C., and preferably has a degradation temperature of at most250° C.

Preferably the concentration of the metal in the poly(hydroxyalkanoate)fiber matrix is at least 0.2 wt %, at least 0.5 wt %, at least 1 wt %,at least 2 wt %, at least 2.5 wt %, at least 3 wt %, at least 5 wt %, atleast 7 wt %, or at least 10 wt %. The concentration of the metal in thepoly(hydroxyalkanoate) fiber matrix may be from 0.1 to 10 wt %, from 0.2to 7 wt %, from 0.5 to 5 wt %, or from 1 to 3 wt %. Preferably the metalis present in the fiber as MgO, CaO, BaO, SrO, tin(II) acetate, tin(II)oxalate, tin(II) octoate, or scandium triflate (Sc(OTf)₃). Morepreferably the metal is present in the fiber as strontium oxide, tin(II)oxalate or tin(II) octoate.

The coating 120 that surrounds at least a portion of the core includes apolymeric material having a degradation temperature higher than thedegradation temperature of the thermally degradable polymeric core 110.When the fiber 100 is heated to the degradation temperature of thethermally degradable polymeric core 110, the core will degrade and canbe removed, providing a hollow fiber having a wall containing thepolymeric material of the coating 120. Preferably, the coating 120 doesnot thermally degrade at temperatures below 250° C. Preferably, thecoating 120 does not thermally degrade at temperatures below 275° C., attemperatures below 280® C., at temperatures below 300® C., attemperatures below 325® C., or at temperatures below 350® C.

The coating 120 may include a polyamide such as nylon; a polyester suchas poly(ethylene terephthalate) and polycaprolactone; a polycarbonate; apolyether; an epoxy polymer; an epoxy vinyl ester polymer; a polyimidesuch as polypyromellitimide (for example KAPTON® (DuPont, Wilmington,Del.); a phenol-formaldehyde polymer such as bakelite(polyoxybenzylmethylenglycolanhydride; an amine-formaldehyde polymersuch as a melamine polymer; a polysulfone; apolyacrylonitrile-butadiene-styrene) (ABS); a polyurethane; a polyolefinsuch as polyethylene, polystyrene, polyacrylonitrile, a polyvinyl,polyvinyl chloride and poly(DCPD); a polyacrylate such as poly(ethylacrylate); a poly(alkylacrylate) such as poly(methyl methacrylate); apolysilane such as poly(carborane-silane); and/or a polyphosphazene.

The coating 120 may include an elastomer, such as an elastomericpolymer, an elastomeric copolymer, an elastomeric block copolymer,and/or an elastomeric polymer blend. Examples of elastomer polymersinclude polyolefins, polysiloxanes such as poly(dimethylsiloxane)(PDMS), polychloroprene, and polysulfides; examples of copolymerelastomers may include polyolefin copolymers and fluorocarbonelastomers; examples of block copolymer elastomers may includeacrylonitrile block copolymers, polystyrene block copolymers, polyolefinblock copolymers, polyester block copolymers, polyamide blockcopolymers, and polyurethane block copolymers; and examples of polymerblend elastomers include mixtures of an elastomer with another polymer.

The coating 120 may include a mixture of these polymers, includingcopolymers that include repeating units of two or more of the polymers,and/or including blends of two or more of the polymers. In one example,the coating 120 includes a blend of at least a first polymer and asecond polymer, where the first polymer has a degradation temperaturebelow 280° C., and the second polymer does not thermally degrade attemperatures below 280® C. In this example, one portion of the coating120 may degrade at a temperature similar to the temperature at which thethermally degradable polymeric core 110 degrades, while another portionof the coating 120 may not degrade at this temperature. When such acoated fiber is heated to the degradation temperature of the firstpolymer of the coating, the thermally degradable polymeric core 110 andthe portion of the coating containing the first polymer can degrade andcan be removed, providing a hollow fiber having a wall containing thesecond polymer of the coating 120.

The coating 120 may include other ingredients in addition to thepolymeric material. For example, the coating may contain one or moreparticulate fillers, stabilizers, antioxidants, flame retardants,plasticizers, colorants and dyes, fragrances, or adhesion promoters. Anadhesion promoter is a substance that increases the adhesion between twosubstances, such as the adhesion between two polymers. One type ofadhesion promoter that may be present includes substances that promoteadhesion between the coating 120 and the thermally degradable polymericcore 110, and substances that promote adhesion between the coating 120and a polymeric matrix in which the partially degradable polymeric fiber100 is contained.

Methods of Making Partially Degradable Polymeric Fibers

FIG. 2 illustrates a schematic representation of an example of a methodof making a partially degradable fiber, such as the partially degradablefiber 100 of FIG. 1. Method 200 includes combining 210 a fiber includinga poly(hydroxyalkanoate) and having an exterior surface, and acomposition including a fluorinated fluid and a metal selected from thegroup consisting of an alkali earth metal and a transition metal. Method200 further includes maintaining 220 the fiber and the compositiontogether at a temperature and for a time sufficient to provide aconcentration of the metal in the fiber of at least 0.1 wt %, separating230 the fiber and the fluorinated fluid, and depositing 240 a layer of amaterial on at least a portion of the exterior surface of the fiber.

An alkali earth metal or a transition metal may be incorporated into apoly(hydroxyalkanoate) fiber through an infusion process, as outlined bythe combining 210, maintaining 220 and separating 230 of method 200. Inone example, PLA fibers may be infused with a tin(II) oxalate (SnOx)catalyst present in an aqueous trifluoroethanol (TFE) mixture. Exposingthe PLA fibers to a solution of TFE:H₂O using a ratio of 60:40 parts byvolume (pbv) with 2% SnOx parts by weight (pbw), for a minimum of 24 hyielded partially degradable polymeric fibers suitable for use in thecomposite formation method of Vaporization of Sacrificial Components(VaSC). The catalyst-containing fibers converted to gas at a lowertemperature and in less time than did pure PLA fibers, as measured byisothermal gravimetric analysis (iTGA), indicating a lowerdepolymerization onset temperature.

FIG. 3 illustrates a schematic representation of another example of amethod of making a partially degradable fiber, such as the partiallydegradable fiber 100 of FIG. 1. Method 300 includes forming 300 aspinning solution including a poly(hydroxyalkanoate), a solvent, and ametal selected from the group consisting of an alkali earth metal and atransition metal, passing 320 the spinning solution through a spinneretto form a fiber containing the poly(hydroxyalkanoate) and the metal andhaving an exterior surface, drying 330 the fiber to provide aconcentration of the metal in the fiber of at least 0.1 wt %, optionallycold-drawing 340 the fiber, and depositing 350 a layer of a material onat least a portion of the exterior surface of the fiber.

An alkali earth metal or a transition metal may be incorporated into apoly(hydroxyalkanoate) fiber through a liquid spinning process, asoutlined by the forming 310, passing 320, drying 330 and optionallycold-drawing 340 of method 300. In one example, a solution of PLA indichloromethane containing 10% SnOx pbw was spun through a 0.5millimeter (mm) spinneret to provide a continuous strand of PLAcontaining the SnOx catalyst. The catalyst-containing fibers formed byliquid spinning converted to gas at a lower temperature and in less timethan did comparable fibers formed by an infusion process, as measured bythermogravimetric analysis (TGA), indicating a lower depolymerizationonset temperature. Cold-drawing the fibers formed from liquid spinningcould increase the fiber strength, ensuring that the fibers can be wovenusing conventional techniques.

Thermally degradable fibers formed by a liquid spinning process, such asthat outlined in FIG. 3, may include a more homogeneous dispersion ofcatalyst within the fiber than do fibers formed by an infusion process,such as that outlined in FIG. 2. An improvement in catalyst distributionprovides for more of the poly(hydroxy-alkanoate) polymer to be in closeproximity to a catalyst species, which in turn can result in a moreefficient depolymerization and a more rapid removal of the fiber.Thermally degradable fibers formed by a liquid spinning process also mayreduce the fabrication time for making the fibers, and may reduce thefabrication time for making a microvascular system from the fibers.While an infusion process can be effective in forming thermallydegradable fibers, the process can require 24 hours for infusing thecatalyst into the fibers, another 24 hours for separating and drying thefibers, and then another 24 hours for degrading and removing the fibersonce a composite is formed that includes the fibers as thermallydegradable polymeric cores. In contrast, thermally degradable fibers maybe formed through liquid spinning within 1 hour, the fibers may be driedwithin 24 hours, and then the fibers may be degraded and removed from acomposite within 2 hours.

A liquid spinning process may be more efficient in its use of catalystthan an infusion process. For example, a spun fiber formed by a liquidspinning process may include a higher concentration of catalyst than aninfused fiber formed by an infusion process, even though the spinningliquid and the infusion liquid include the same initial concentration ofcatalyst. Thus, to achieve a given loading of catalyst in a thermallydegradable fiber, a liquid spinning process may require less totalcatalyst than a comparable infusion process.

Thermally degradable fibers formed by a liquid spinning process mayinclude a wider variety of depolymerization catalysts than can beincluded using an infusion process. In one example, infusion of PLAfibers with tin(II) octoate (SnOc) provided fibers with a greasysurface, whereas liquid spinning provided PLA fibers containing SnOc,but with a more desirable non-greasy surface. As the depolymerizationtemperature of PLA fibers containing SnOc is lower than that of PLAfibers containing SnOx, the liquid spinning method can provide PLAfibers that are readily incorporated into a composite and thatdepolymerize at a relatively low temperature.

A method of making a thermally degradable fiber may include other knownmethods of incorporating an additive into a polymer fiber, such as meltspinning. In the example of melt spinning, the temperature of thematerial should be maintained below 180° C., the temperature at whichPLA can depolymerize in the presence of a catalyst containing an alkaliearth metal or a transition metal. One potential advantage of meltspinning PLA fibers containing a depolymerization catalyst is that thefibers may be stronger than comparable fibers formed by infusion or byliquid spinning.

The depositing (240, 350) a layer of a material on at least a portion ofthe exterior surface of a fiber may include any of a variety of methodsfor depositing a coating material on a fiber. The depositing may includecontacting the exterior surface of the thermally degradable fiber with acoating precursor, and then solidifying the coating precursor to form asolid coating on at least a portion of the exterior surface of thefiber. A coating precursor may be any substance that can form a solidpolymeric material when solidified. The coating precursor may besubstantially homogeneous, or it may include other substances, such asfillers and/or viscosity modifiers.

In one example, a coating precursor includes a monomer and/or prepolymerthat can polymerize to form a polymer, such as a polymer as describedabove with regard to coating 120. The coating precursor may then besolidified by polymerizing the monomer and/or prepolymer of theprecursor to form the solid polymeric coating. Examples of monomersand/or prepolymers that can polymerize to form a polymer include cyclicolefins; unsaturated monomers such as acrylates, alkylacrylates(including methacrylates and ethacrylates), styrenes, isoprene andbutadiene; lactones (such as caprolactone); lactams;epoxy-functionalized monomers, prepolymers or polymers; functionalizedsiloxanes; and two-part precursors for polymers such as polyethers,polyesters, polycarbonates, polyanhydrides, polyamides, formaldehydepolymers (including phenol-formaldehyde, urea-formaldehyde andmelamine-formaldehyde), and polyurethanes. Polymerization of a coatingprecursor may include crosslinking of monomers and/or prepolymers toform an insoluble polymer network. Crosslinking may be performed by avariety of methods, including the addition of chemical curing agentsand/or exposure to radiation such as infrared radiation (IR; i.e. heat),visible light, or ultraviolet radiation (UV).

In another example, the coating precursor includes a polymer in asolvent. The polymer may be dissolved or dispersed in the solvent toform the coating precursor. The coating precursor may be solidified byremoving at least a portion of the solvent from the composition to formthe solid polymeric coating.

The depositing (240, 350) a layer of a material on at least a portion ofthe exterior surface of the fiber may further include applying a surfacetreatment to the solid polymeric coating. Examples of surface treatmentsinclude functionalization of the coating by contacting the coating withan oxidizing or reducing atmosphere or by contacting the coating with aliquid containing a functionalizing reagent. Examples of surfacetreatments include applying an adhesion promoter to the coating.

FIG. 4A depicts a schematic representation of an apparatus 400 fordepositing a coating on at least a portion of the exterior surface of athermally degradable fiber. A thermally degradable fiber 402 is unwoundfrom a bundle 404 using a tensioner 410, and then passed through a bath420 containing a coating precursor. A layer of the precursor isdeposited on at least a portion of the exterior surface of the fiber 402in the bath 420, and the precursor and the fiber are then heated by aheater 430. The precursor on the fiber is solidified due to the heating,providing a coated fiber 406, which is collected on a take-up drum 440.Preferably the coating precursor is solidified at a temperature belowthe degradation temperature of the thermally degradable fiber 402. Thesolidification of the coating precursor to form a coating on the fibermay include removal of solvent from the precursor and/or chemicalreaction (i.e. curing) of the precursor. Examples of solidificationtemperatures include 30° C., 50° C., 75° C., 100° C., 125° C., 150° C.and 180° C.

In one example, the coating precursor in the bath 420 includes a mixtureof a polyimide in a solvent. When the solvent is removed, the resultingcoated fiber 406 includes the thermally degradable fiber 402 and apolyimide coating surrounding at least a portion of the thermallydegradable fiber. In another example, the coating precursor in the bath420 includes a mixture of a polyimide and a thermally degradablepoly(hydroxyalkanoate) such as PLA in a solvent. When the solvent isremoved, the resulting coated fiber 406 includes the thermallydegradable fiber 402 and a coating containing both a polyimide and thepoly(hydroxyalkanoate), surrounding at least a portion of the thermallydegradable fiber.

FIG. 4B depicts a schematic representation of an apparatus 450 fordepositing a coating on at least a portion of the exterior surface of athermally degradable fiber. A thermally degradable fiber 452 is unwoundfrom a bundle 454 using a tensioner 460, and then passed through a bath470 containing a coating precursor. A layer of the precursor isdeposited on at least a portion of the exterior surface of the fiber 452in the bath 470, and the precursor and the fiber is then irradiated by aradiation source 480. The precursor on the fiber is cured due to theirradiation, providing a coated fiber 456, which is collected on atake-up drum 490. Preferably the irradiation does not cause degradationof the thermally degradable fiber 452.

In one example, the coating precursor in the bath 470 includes a mixtureof a polysiloxane precursor and an ultraviolet (UV) initiator in asolvent. When the polysiloxane is cured, the resulting coated fiber 456includes the thermally degradable fiber 452 and a polysiloxane coatingsurrounding at least a portion of the thermally degradable fiber.

Composite Materials Including Partially Degradable Polymeric Fibers

FIG. 5 depicts a schematic representation of a composite material 500that includes a solid polymeric matrix 510 and a partially degradablefiber 520 in the polymeric matrix. The solid polymeric matrix 510includes a first material. The partially degradable fiber 520 includes athermally degradable polymeric core 522 and a coating 524 surrounding atleast a portion of the core. The coating 524 includes a second material,which is different from the first material of the matrix 510.

The solid polymer matrix 510 may include a polyamide such as nylon; apolyester such as poly(ethylene terephthalate) and polycaprolactone; apolycarbonate; a polyether; an epoxy polymer; an epoxy vinyl esterpolymer; a polyimide such as polypyromellitimide (for example KAPTON®);a phenol-formaldehyde polymer such as bakelite; an amine-formaldehydepolymer such as a melamine polymer; a polysulfone; apoly(acrylonitrile-butadiene-styrene) (ABS); a polyurethane; apolyolefin such as polyethylene, polystyrene, polyacrylonitrile, apolyvinyl, polyvinyl chloride and poly(DCPD); a polyacrylate such aspoly(ethyl acrylate); a poly(alkylacrylate) such as poly(methylmethacrylate); a polysilane such as poly(carborane-silane); and/or apolyphosphazene.

The solid polymer matrix 510 may include an elastomer, such as anelastomeric polymer, an elastomeric copolymer, an elastomeric blockcopolymer, and/or an elastomeric polymer blend. Examples of elastomerpolymers include polyolefins, polysiloxanes such aspoly(dimethylsiloxane) (PDMS), polychloroprene, and polysulfides;examples of copolymer elastomers may include polyolefin copolymers andfluorocarbon elastomers; examples of block copolymer elastomers mayinclude acrylonitrile block copolymers, polystyrene block copolymers,polyolefin block copolymers, polyester block copolymers, polyamide blockcopolymers, and polyurethane block copolymers; and examples of polymerblend elastomers include mixtures of an elastomer with another polymer.Composite materials that include an elastomer as the solid polymermatrix are disclosed, for example, in U.S. Pat. No. 7,569,625 to Kelleret al., and in U.S. Application Publication 2009/0191402 to Beiermann etal., which are incorporated by reference. The solid polymer matrix 510may include a mixture of these polymers, including copolymers thatinclude repeating units of two or more of these polymers, and/orincluding blends of two or more of these polymers.

The solid polymer matrix 510 may include other ingredients in additionto the polymeric material. For example, the matrix may contain one ormore particulate fillers, stabilizers, antioxidants, flame retardants,plasticizers, colorants and dyes, fragrances, or adhesion promoters. Anadhesion promoter is a substance that increases the adhesion between twosubstances, such as the adhesion between two polymers. One type ofadhesion promoter that may be present includes substances that promoteadhesion between the solid polymer matrix 510 and the coating 524.

The partially degradable fiber 520, including the thermally degradablepolymeric core 522 and the coating 524 surrounding at least a portion ofthe core, may be as described above for the partially degradable fiber100. The partially degradable fiber 520 may be configured in a varietyof shapes, including a woven shape. In one example, composite 500includes a woven structure in the solid polymer matrix 510, where thewoven structure includes a plurality of partially degradable fibers 520that are interwoven. In another example, composite 500 includes a wovenstructure in the solid polymer matrix 510, where the woven structureincludes a first plurality of partially degradable fibers, and a secondplurality of partially degradable fibers. In this example, eachplurality of partially degradable fibers may be as described above forthe partially degradable fiber 520. In yet another example, composite500 includes a woven structure in the solid polymer matrix 510, wherethe woven structure includes a plurality of partially degradable fibers520, and a plurality of reinforcing fibers.

When the composite 500 is heated to and maintained at a temperature offrom 100 to 250° C., the thermally degradable polymeric core 522 canform degradants that can be removed from the composite 500. Completedegradation of the core 522 and removal of the resulting degradantsyields a microvascular system containing the solid polymeric matrix 510and a microfluidic channel, where the walls of the microfluidic channelare provided by the coating 524.

Microvascular Systems Formed from Partially Degradable Polymeric Fibers

FIG. 6 depicts a schematic representation of a microvascular system 600,which includes a solid polymeric matrix 610 and a woven structure 620 inthe matrix. The woven structure 620 includes at least one ply containinga plurality of microfluidic channels 630. The microfluidic channels 630have a channel wall including a material that is different from thematerial of the solid polymeric matrix 610. The woven structure 620optionally includes a plurality of structures 640, which may include aplurality of fibers and/or a second plurality of microfluidic channels.

The woven structure 620 includes at least one ply containing at leastone plurality of microfluidic channels 630, and may include more thanone ply containing microfluidic channels. For a microvascular systemthat includes the optional second plurality of microfluidic channels(i.e. 640), the woven structure 620 also may include at least one plycontaining only the first plurality of microfluidic channels 630 and/ormay include at least one ply containing only the second plurality ofmicrofluidic channels 640. Each plurality of microfluidic channels haschannel walls including materials that are different from the materialof the solid polymeric matrix 610.

The microfluidic channels 630 may include a fluid, such as a gas or aliquid, or they may include a vacuum. The microvascular system 600 maybe referred to as an “apomatrix” when the microfluidic channels 630include a fluid. Without fluid, or with air, microvascular system 600may be referred to as a “holomatrix”.

A variety of fluids may be present in the microfluidic channels of amicrovascular system such as system 600, including aqueous solutions,organic solvents, liquid metals, reactive gases and inert gases. A fluidin the microfluidic channels 630 can flow through one channel and intoanother channel by way of an interconnect between the channels. Aninterconnect is present wherever two or more channels are in contactwithout a channel wall separating the channels. In this manner,interconnects establish fluid communication between the channels.Microfluidic channels 630 that are interconnected thus form amicrofluidic network. If the polymeric matrix includes an inlet port, afluid delivered through the inlet port can flow through interconnectedmicrofluidic channels 630 within the polymeric matrix. If theinterconnected microfluidic channels form a network, the introducedfluid may at least partially fill the network.

For a microvascular system that includes the optional second pluralityof microfluidic channels (i.e. 640), the first plurality of microfluidicchannels 630 and the second plurality of microfluidic channels 640 maybe independent, or they may be in fluid communication. Fluidcommunication between the first and second pluralities of microfluidicchannels may be provided by interconnects between channels. Fluidcommunication between the first and second pluralities of microfluidicchannels also may be provided by channels that are in contact, even ifthe channels are separated by one or more channel walls. The latter typeof fluid communication is provided by permeation of a fluid through thechannel wall(s).

The nature and/or level of any fluid communication between the first andsecond plurality of microfluidic channels is affected by the relativeconfigurations of the microfluidic channels. For a first and secondplurality of channels that are in fluid communication, it is preferredthat at least one channel of each of the pluralities are in physicalcontact with each other. If the channels are not in physical contact,fluid communication would require permeation of a fluid through thesolid polymer matrix, in addition to permeation through the channelwall(s).

The nature and/or level of any fluid permeation between the first andsecond plurality of microfluidic channels is affected by the propertiesof their channel walls. In one example, the channel walls of themicrofluidic channels 630 are impermeable to a fluid within thechannels. An impermeable channel wall may allow substantially notransport of a liquid through the channel wall. For example, the channelwall may include a liquid barrier polymer such as polyethylene,polypropylene, polystyrene, polyamide, polyester, polycarbonate (PC),poly(methyl methacrylate) (PMMA), or poly(vinyl chloride) (PVC). Animpermeable channel wall may allow substantially no transport of a gasthrough the channel wall. For example, the channel wall may include agas barrier polymer such as isobutylene-isoprene rubber, bromobutylrubber, chlorobutyl rubber, or poly(tetrafluoroethylene) (PTFE) or otherfluoroelastomers.

In another example, the channel walls of the microfluidic channels 630are permeable to a fluid within the channels. A permeable channel wallmay allow a liquid and/or a gas to be transported through the channelwall. For example, the channel wall may include a porous polymericmaterial conventionally used for ultrafiltration membranes,microfiltration membranes, nanofiltration membranes, or reverse osmosismembranes. Such porous materials may be formed from polymers such ascellulose acetate, polyethylene, polypropylene, PVC, polysulfone,polyamide, polyimide, polysulfone, polyether sulfone, poly(vinylidenefluoride) (PVDF), polyacrylonitrile (PAN), and ion-exchange membranesuch as a sulfonated PTFE (for example NAFION® (DuPont)).

The permeability properties of a channel wall material may depend on thechemical nature of the fluid. A channel wall is semi-permeable when itis permeable to some fluids, but impermeable to other fluids. Forexample, a hydrophobic polymer may be impermeable with regard to aqueousliquids, but may have some permeability with regard to lipophilicliquids such as hydrocarbons. Examples of hydrophobic polymers includepolyethylene, polypropylene, PVC, polysulfone, and PTFE. In anotherexample, a hydrophilic polymer may be impermeable with regard tolipophilic liquids, but may have some permeability with regard toaqueous liquids. Examples of hydrophilic polymers include cellulosepolymers and poly(vinyl alcohol).

As noted above, the woven structure 620 includes at least one plycontaining at least one plurality of microfluidic channels 630. Thewoven structure 620 may include more than one ply containingmicrofluidic channels. For a microvascular system that includes theoptional fibers (i.e. 640), the woven structure 620 also may include atleast one ply containing a plurality of fibers without any microfluidicchannels and/or may include at least one ply containing a plurality ofmicrofluidic channels 630 without any fibers. The fibers may be presentas tows, also referred to as yarns, which are assemblies of from 100 to12,000 individual fibers.

Optional fibers (i.e. 640) may include a material having an aspect ratio(diameter:length) of at least 1:10, including at least 1:100 and atleast 1:1,000. If present, the optional fibers preferably includereinforcing fibers that, when added to a solid polymer matrix, increasethe strength of the matrix relative to the pure polymer. Reinforcingfibers may include an inorganic and/or an organic material. Examples offibrous reinforcing materials include graphite fibers, ceramic fibers,metal fibers, and polymer fibers. Examples of graphite reinforcingfibers include THORNEL® 25 (Cytec, Tempe, Ariz.) and MODMOR® (Morganite,United Kingdom). Examples of ceramic reinforcing fibers include metaloxide fibers such as titanium oxide fibers, zirconium oxide fibers andaluminum oxide fibers; silica fibers; and glass fibers, such as E-glassfibers and S-glass fibers. Examples of metal fibers include steelfibers, tungsten fibers, beryllium fibers, and fibers containing alloysof these metals. Examples of polymer fibers include polyester fibers,nylon fibers, rayon fibers, and polyaramid fibers, such as KEVLAR® 49(DuPont).

The woven structure 620 may be a two-dimensional (2D) structure, inwhich the ply includes threads oriented in two different directions insubstantially a single plane. The woven structure 620 may be athree-dimensional (3D) structure, in which the ply includes threadsoriented in two different directions in substantially a single plane,and further includes threads oriented in a third direction that issubstantially orthogonal to the plane. An individual “thread” in thewoven structure 620 may be a microfluidic channel 630 or a structure640, which may be an individual fiber, a fiber tow, or a microfluidicchannel of a second plurality of channels.

The microfluidic channels 630 (and optionally 640) may includesubstantially tubular channels having a diameter less than 1,000micrometers. The term “substantially tubular” means that the majority ofthe cross-sectional periphery of the channel through the substratematrix is curved in shape. Curved can include circular, elliptic,rounded, arched, parabolic and other curved shapes. The average diameterof the substantially tubular channels preferably is from 0.1 to 1,000micrometers, from 10 to 750 micrometers, from 20 to 500 micrometers, orfrom 50 to 250 micrometers. The microfluidic channels 630 may have alength of at least 1 centimeter.

At least a portion of the microfluidic channels 630 can be independent,existing in the matrix 610 without any interconnect with anotherchannel. In one example, all of the microfluidic channels 630 in amicrovascular system 600 are independent, and the system does notinclude a microfluidic network. In this example, any fluid in anindividual microfluidic channel 630 is not in fluid communication with afluid in another microfluidic channel.

The microfluidic channels 630 in the polymeric matrix 610 can affect thestructural properties of the matrix, and the type and magnitude of theresulting structural property changes may depend on the properties ofthe channels and their configuration in the matrix. For example, it maybe desirable for the microfluidic channels 630 to have a minimum channelspacing and a maximum channel diameter, which may help to minimize anydecrease in the strength of the matrix.

Methods of Making a Microvascular System from Partially DegradablePolymeric Fibers

FIG. 7 illustrates a schematic representation of an example of a methodof making a microvascular system. Method 700 includes forming 710 acomposite that includes a solid polymeric matrix and a plurality ofpartially degradable fibers in the matrix, where the partiallydegradable fibers include a thermally degradable polymeric core and acoating surrounding the core. Method 700 further includes heating 720the composite to a temperature of from 100 to 250° C., maintaining 730the composite at a temperature of from 100 to 250° C. for a timesufficient to form degradants from the polymeric cores, and removing 740the degradants from the composite to provide microfluidic channels.Method 700 optionally further includes introducing 750 a fluid into atleast a portion of the microfluidic channels.

Forming 710 a composite that includes a solid polymeric matrix and aplurality of partially degradable polymeric fibers in the matrix mayinclude combining a matrix precursor with a plurality of partiallydegradable polymeric fibers, and then solidifying the matrix precursorto form a solid polymer matrix. The method may further include formingthe partially degradable polymeric fibers and/or the matrix precursor.

The matrix precursor may be any substance that can form a solid polymermatrix when solidified. The matrix precursor may be substantiallyhomogeneous, or it may include other substances, such as fillers and/orviscosity modifiers. For example, a matrix precursor may includeparticles that can change the viscosity of the precursor and/or canchange the properties of the polymeric matrix formed from the precursor.Examples of particles that may be present in the matrix precursorinclude plastic particles and non-plastic particles, such as ceramics,glasses, semiconductors, and metals.

In one example, the matrix precursor includes a monomer and/orprepolymer that can polymerize to form a polymer. The sacrificial fibersand optionally other ingredients may be mixed with the monomer orprepolymer. The matrix precursor may then be solidified by polymerizingthe monomer and/or prepolymer of the matrix precursor to form the solidpolymer matrix.

Examples of monomers and/or prepolymers that can polymerize to form apolymer include cyclic olefins; unsaturated monomers such as acrylates,alkylacrylates (including methacrylates and ethacrylates), styrenes,isoprene and butadiene; lactones (such as caprolactone); lactams;epoxy-functionalized monomers, prepolymers or polymers; functionalizedsiloxanes; and two-part precursors for polymers such as polyethers,polyesters, polycarbonates, polyanhydrides, polyamides, formaldehydepolymers (including phenol-formaldehyde, urea-formaldehyde andmelamine-formaldehyde), and polyurethanes. Polymerization of a matrixprecursor may include crosslinking of monomers and/or prepolymers toform an insoluble polymer network. Crosslinking may be performed by avariety of methods, including the addition of chemical curing agents,exposure to light or other forms of radiation, or heating. If a chemicalcuring agent is used, it may be added to the matrix precursor before orafter it is combined with the sacrificial fibers.

In another example, the matrix precursor includes a polymer in a matrixsolvent. The polymer may be dissolved or dispersed in the matrix solventto form the matrix precursor, and the sacrificial fibers and optionallyother ingredients then mixed into the matrix precursor. The matrixprecursor may be solidified by removing at least a portion of the matrixsolvent from the composition to form the solid polymer matrix.

In another example, the matrix precursor includes a polymer that is at atemperature above its melting temperature. The polymer may be melted toform the matrix precursor and then mixed with the sacrificial fibers andoptionally other ingredients. The matrix precursor may be solidified bycooling the composition to a temperature below the melt temperature ofthe polymer to form the solid polymer matrix.

Forming 710 preferably includes contacting the partially degradablepolymeric fibers with a matrix precursor at a temperature below thedegradation temperature of the thermally degradable polymeric core.Preferably the partially degradable polymeric fibers are contacted witha matrix precursor at a temperature between 30° C. and 250° C.,including temperatures between 50° C. and 200° C., between 75° C. and175° C., and between 100° C. and 150° C. In one example, forming 710includes contacting the partially degradable polymeric fibers with amatrix precursor that includes a monomer and/or prepolymer, and heatingthe matrix precursor and the partially degradable polymeric fibers for atime sufficient to polymerize the monomer and/or prepolymer. In anotherexample, forming 710 includes contacting the partially degradablepolymeric fibers with a matrix precursor that includes a polymer that isat a temperature above its melting temperature.

The partially degradable fibers include a thermally degradable polymericcore having a degradation temperature of at most 250° C., and a coatingsurrounding at least a portion of the core, where at least a portion ofthe coating does not thermally degrade at temperatures below 275° C. Thepartially degradable fibers may be as described above for partiallydegradable fiber 100.

Heating 720 the composite to a temperature of from 100 to 250° C. andmaintaining 730 the composite at a temperature of from 100 to 250° C.for a time sufficient to form degradants from the thermally degradablepolymeric cores of the partially degradable polymeric fibers mayinclude, for example, placing the composite in an oven. The degradantspreferably have an average molecular weight less than 500 Daltons, andpreferably are in a gas phase.

Removing 740 the degradants from the composite may include contacting atleast a portion of a surface of the composite with a vacuum source.Removing 740 the degradants from the composite may include contacting atleast a portion of a surface of the composite with a pressurized fluid,such as a gas. Use of a pressurized fluid or a vacuum may facilitateremoval of gaseous degradants. The composite may be maintained at atemperature of from 100 to 250° C. during the removal, or thetemperature of the composite may be raised or lowered prior to or duringthe removal. Removing 740 the degradants from the composite may occursimultaneously with the heating 720 and/or maintaining 730 of thecomposite, or the removing may occur after the maintaining 730 of thecomposite.

Optionally introducing 750 a fluid into at least a portion of themicrofluidic channels may include any of a variety of methods forintroducing a fluid into a microfluidic channel. In one example, thefluid may be injected into one or more channels. In another example, oneor more channel openings may be placed in contact with a reservoir ofthe fluid. The fluid may then flow through the channels throughcapillary action.

FIG. 8 depicts a schematic representation of a composite 890, whichincludes a solid polymeric matrix 810 and a plurality of partiallydegradable fibers 850, and of a composite 800, which includes thepolymeric matrix 810 and a plurality of microfluidic channels 840. Thepartially degradable fiber 850 includes a thermally degradable polymericcore 860 and a coating 870 surrounding at least a portion of the core.In FIG. 8, the thermally degradable polymeric core 860 is beingconverted into degradants 865 that are subsequently removed, forming themicrofluidic channel 840 having wall 880 provided by the coating 870.Composite 890 may be the product of the forming 710 of method 700 ofFIG. 7, for example. Composite 800 may be the product of the heating720, maintaining 730 and removing 740 of method 700 of FIG. 7, forexample.

The solid polymer matrix 810 may include a polymeric material, and mayinclude other ingredients in addition to the polymeric material, asdescribed above for solid polymer matrix 510 of FIG. 5. The microfluidicchannels 540 may have the dimensions and configuration as describedabove for microfluidic channels 540.

The partially degradable fiber 850 should be strong enough to survive amechanical weaving process to survive being combined with a matrixprecursor. The partially degradable fiber 850 also should remain solidduring solidification of the matrix precursor into a polymeric matrix.For solidification by polymerization and/or curing, the partiallydegradable fiber 850 preferably should remain solid at temperatures upto 180° C. The thermally degradable polymeric core 860 of the partiallydegradable fiber 850 also should be easily removed from a polymericmatrix by degradation to volatile degradants at higher temperatures. Thethermally degradable polymeric core 860 also should have degradation andvolatilization temperatures within a narrow range between the highestmatrix solidification temperatures and the lowest thermal degradationtemperatures of the polymeric matrix (200-240° C.). Preferably, thedegradation temperature of the thermally degradable polymeric core 860is at most 250° C. More preferably, the degradation temperature of thecore is at most 220° C., and more preferably is at most 180° C.

FIG. 9 depicts a schematic representation of a microvascular system 900,which includes a solid polymeric matrix 910, a first microfluidicchannel 920 in the solid polymeric matrix, and a second microfluidicchannel 930 in the solid polymeric matrix. The first microfluidicchannel 920 has a first channel wall 924 including a polymer that isdifferent from the solid polymeric matrix 910, and the secondmicrofluidic channel 930 has a second channel wall 934 including apolymer that is different from the solid polymeric matrix 910.

The first channel 920 may include a first fluid, and the second channel930 may include a second fluid different from the first fluid. Aschannels 920 and 930 are in physical contact, the level of communicationbetween the first and second fluids is determined by the permeabilitiesof the channel walls 924 and 934. If both channel walls are permeable toboth fluids, then the first and second fluids can combine, forming amixture of the fluids in each channel. If both channel walls arepermeable only to the first fluid and/or to an ingredient of the firstfluid, then the first fluid may permeate through the channel walls andcombine with the second fluid. In this example, the second channel 930can contain a mixture of the first and second fluids, while the firstchannel 920 is depleted of the first fluid and/or maintains the firstfluid at a reduced fluid pressure. If both channel walls are impermeableto both fluids, then the first and second fluids can flow through theirrespective microfluidic channels without mixing with each other. Thefirst and second channel walls 924 and 934 may include the same polymer,or they may include different polymers.

The microvascular system 900 may be formed from a composite material 940including the solid polymeric matrix 910, a first partially degradablefiber 950 in the solid polymeric matrix, and a second partiallydegradable fiber 960 in the solid polymeric matrix. The first partiallydegradable fiber 950 includes a first thermally degradable polymericcore 952 and a first coating 954 surrounding at least a portion of thecore. The second partially degradable fiber 960 includes a secondthermally degradable polymeric core 962 and a first coating 964surrounding at least a portion of the core. The microvascular system 900may be formed from the composite material 940 by heating the compositeto a temperature of from 100 to 250° C., maintaining the composite at atemperature of from 100 to 250° C. for a time sufficient to formdegradants from the thermally degradable polymeric cores 952 and 962,and removing the degradants from the composite to provide microfluidicchannels 920 and 930.

The first and second thermally degradable polymeric cores 952 and 962may include the same polymer, or they may include different polymers.Preferably the first and second cores are the same material, as this canprovide for removal of both cores simultaneously. The first and secondcoatings 924 and 934 may include the same polymer, or they may includedifferent polymers.

The composite material 940 may be formed by placing the first partiallydegradable fiber 950 and the second partially degradable fiber 960 in adesired configuration, contacting the fibers with a matrix precursor,and solidifying the matrix precursor to form the polymeric matrix 910.The partially degradable fibers (i.e. 950) may be formed by depositing acoating 954 on at least a portion of an external surface of a thermallydegradable polymeric fiber 952.

FIG. 10 illustrates a schematic representation of an example of a methodof making a microvascular system, such as microvascular system 600 ofFIG. 6 or microvascular system 900 of FIG. 9. Method 1000 includesforming 1010 a composite that includes a solid polymeric matrix and awoven structure in the matrix, where the woven structure includes aplurality of partially degradable polymeric fibers. The partiallydegradable fibers include a thermally degradable polymeric core, and acoating surrounding at least a portion of the core. Method 1000 furtherincludes heating 1020 the composite to a temperature of from 100 to 250°C., maintaining 1030 the composite at a temperature of from 100 to 250°C. for a time sufficient to form degradants from the thermallydegradable polymeric cores, and removing 1040 the degradants from thecomposite to provide microfluidic channels. The degradants preferablyhave an average molecular weight less than 500 Daltons. Method 1000optionally further includes introducing 1050 a fluid into at least aportion of the microfluidic channels.

Forming 1010 a composite that includes a solid polymeric matrix and awoven structure in the matrix may include combining a matrix precursorwith the partially degradable polymeric fibers, and then solidifying thematrix precursor to form a solid polymer matrix. The method may furtherinclude forming the partially degradable polymeric fibers and/or thematrix precursor.

Forming 1010 may include forming the woven structure by weaving theplurality of partially degradable polymeric fibers to form a single ply.In one example, an arrangement of warp threads in a first orientationmay be held in tension, and weft threads then may be directedsinusoidally in a second orientation through the warp threads.Preferably the second direction is transverse to the first orientation.In this example, the resulting ply is a 2D woven structure.

In another example, an arrangement of warp threads in a firstorientation may be held in tension. Weft threads then may be directed ina second orientation over, under and/or through the warp threads, wherethe second direction preferably is transverse to the first orientation.Z-threads then may be directed through the warp and weft threads,preferably in an orientation that is orthogonal to a plane formed by theweft and warp threads. The Z-threads may be directed through the weftand warp threads sinusoidally. The warp, weft and/or Z-threads mayinclude partially degradable polymeric fibers. In this example, theresulting ply is a 3D woven structure.

Forming 1010 may include forming the woven structure by weaving theplurality of partially degradable polymeric fibers with a secondplurality of partially degradable polymeric fibers and/or withreinforcing fibers to form a single ply. In one example, the warpthreads may include the plurality of partially degradable polymericfibers, and the weft threads may include a second plurality of partiallydegradable polymeric fibers and/or reinforcing fibers. In anotherexample, the warp threads may include a second plurality of partiallydegradable polymeric fibers and/or reinforcing fibers, and the weftthreads may include the plurality of partially degradable polymericfibers. In these examples, the resulting plies are 2D woven structures.

In another example, an arrangement of warp threads in a firstorientation may be held in tension, weft threads then may be directed ina second orientation over, under and/or through the warp threads, wherethe second direction preferably is transverse to the first orientation,and Z-threads then may be directed through the warp and weft threads,preferably in an orientation that is orthogonal to a plane formed by theweft and warp threads. The Z-threads may be directed through the weftand warp threads sinusoidally. At least one of the warp, weft and/orZ-threads may include the partially degradable polymeric fibers, and theremaining threads may include a second plurality of partially degradablepolymeric fibers and/or reinforcing fibers. In this example, theresulting ply is a 3D woven structure.

Forming 1010 may include inserting the plurality of partially degradablepolymeric fibers into a ply of woven a second plurality of partiallydegradable polymeric fibers and/or reinforcing fibers. In one example, apartially degradable polymeric fiber is stitched into a woven ply offibers, such as by repeatedly piercing the ply with a needle attached toa partially degradable polymeric fiber, and forming a sinusoidal traceof the partially degradable polymeric fiber that traverses the thicknessof the ply. In this example, a pattern of the first plurality ofpartially degradable polymeric fibers may be formed along the length andwidth of the woven ply.

In one example, the plurality of partially degradable polymeric fibersand a second plurality of partially degradable polymeric fibers and/orreinforcing fibers may be arranged into two- or three-dimensional wovenpreforms. The position, length, diameter, and curvature of the partiallydegradable polymeric fibers and/or reinforcing fibers may be varied tomeet desired design criteria.

Forming 1010 includes combining the partially degradable polymericfibers and a matrix precursor. The matrix precursor may be as describedwith regard to forming 710 of FIG. 7. Forming 1010 preferably includescontacting the partially degradable polymeric fibers with a matrixprecursor and heating the matrix precursor to a temperature of at least75° C. for a time sufficient to form the polymeric matrix. In oneexample, forming 1010 includes infiltrating the interstitial pore spacebetween fibers with a low-viscosity thermosetting resin (e.g. epoxy) andcuring at elevated temperature.

After curing, the sample may be trimmed to expose the ends of thepartially degradable polymeric fibers.

Heating 1020 the composite to a temperature of from 100 to 250° C. andmaintaining 1030 the composite at a temperature of from 100 to 250° C.for a time sufficient to form degradants from the thermally degradablepolymeric cores may include, for example, placing the composite in anoven. The degradants preferably have an average molecular weight lessthan 500 Daltons, and preferably are in a gas phase. Removing 1040 thedegradants from the composite may include contacting at least a portionof a surface of the composite with a vacuum source or with a pressurizedfluid. The heating 1020, maintaining 1030 and removing 1040 may be asdescribed above for heating 720, maintaining 730 and removing 740 ofFIG. 7. In one example, the heating 1020 may be performed above 200° C.,and the maintaining 1030 and subsequent removing 1040 may provide emptychannels and at least one 3D vascular network throughout the composite.

Optionally introducing 1050 a fluid into at least a portion of themicrofluidic channels may include any of a variety of methods forintroducing a fluid into a microfluidic channel, as described above forintroducing 750 of FIG. 7. In one example, a microvascular composite isfilled with at least one fluid having the desired physical properties tocreate a multifunctional material.

The presence of partially degradable polymeric fibers in a woven fiberpreform can provide seamless fabrication of microvascular compositesthat are both strong and multifunctional. Preferably the hollow channelsproduced in the composites are high-fidelity inverse replicas of theoriginal fibers' diameters and trajectories. Use of methods 700 and/or1000 can provide microvascular fiber-reinforced composites with channelsover one meter in length that then can be filled with a variety offluids including aqueous solutions, organic solvents, and liquid metals.

Methods 700 and 1000 are examples of a method referred to asVaporization of Sacrificial Components (VaSC). The VaSC methods canprovide composite materials that include microfluidic channels having arange of channel curvatures and diameters, allowing the construction ofa wide variety of network architectures. The methods also can providecomposite materials that include microfluidic channels that areinterconnected and/or branched.

When incorporated into a matrix of another material, a thermallydegradable polymeric core containing a poly(hydroxyalkanoate) includingan alkali earth metal or a transition metal, where the concentration ofthe metal in the fiber matrix is at least 0.1 wt %, preferably may beremoved by heating at 200° C. The heating and removal may occur over thecourse of several minutes to several hours. Preferably the heating andremoval are completed in at most 24 hours, at most 5 hours, at most 3hours, or at most 2 hours. Details regarding the formation, degradationand removal of poly(hydroxyalkanoate) fibers is described, for example,in U.S. patent application Ser. No. 13/416,002, filed Mar. 9, 2012, withinventors Esser-Kahn et al., which is incorporated herein by reference.

The clearing of lactide from channels formed by degradation of PLAfibers including an alkali earth metal or a transition metal typicallymay result in a very low number of obstructions. Hidden defects in thechannels may be present, and may be caused by complex channelgeometries. Defects may be detected by calculating a theoretical valuefor pressure drop according to the Hagen-Pouiselle relation andcomparing the prediction with a measured pressure head for the channels.A negligible difference from between these values indicates geometricuniformity and substantially complete channel clearing.

Partially degradable polymeric fibers having a thermally degradablepolymeric core that includes a poly(hydroxyalkanoate) including analkali earth metal or a transition metal, where the concentration of themetal in the fiber matrix is at least 0.1 wt %, preferably arecompatible with fiber preform fabrication. Preferably the single fibertension strength of a partially degradable polymeric fiber exceeds thethreshold stress of 23 MPa applied during automated weaving. Preferablythe single fiber tension strength of a partially degradable polymericfiber is at least 30 MPa, at least 50 MPa, at least 75 MPa, or at least100 MPa.

Uses of Microvascular Systems

Microvascular networks capable of independent fluid flow and/orcontrolled mixing between fluids are relevant for a range ofapplications in microfluidics and self-healing systems. A variety ofproperties may be obtained with a single microvascular system byselection of one or more fluids for introduction to the microchannels.The variation in properties can be obtained without varying thecomposite's form factor. Examples of materials properties that may beaffected by the fluid in the microchannels of the composites includethermal management, electro-magnetic signature, electrical conductivitytuning, and chemical reactivity.

A microvascular system that includes independent microfluidic channelshaving independent fluid flow may include modified surfaces within someor all of the microfluidic channels. For example, one plurality ofinterconnected microfluidic channels in a system may have a hydrophilicsurface at the interior of its channel walls, while another plurality ofinterconnected microfluidic channels in a system may have a hydrophobicsurface at the interior of its channel walls. Hydrophilic and/orhydrophobic surfaces at the interior of microfluidic channel walls maybe formed by any of a variety of known techniques, including thoselisted above with regard to applying a surface treatment to the solidpolymeric coating as part of the depositing (240, 350). Hydrophilicand/or hydrophobic surfaces at the interior of microfluidic channelwalls may be formed using photocleavable self-assembled monolayers(SAMs) as described in Zhao et al., J. Am. Chem. Soc. 124(19), 2002,5284-85. It may also be desirable to modify the interior of microfluidicchannels in microvascular systems that do not include independentchannels.

FIG. 11 depicts a microvascular system 1100, which includes a solidpolymeric matrix 1110 and a microchannel structure 1120 in the matrix.The microchannel structure 1120 includes independent microfluidicchannels 1130, 1140, 1150, 1160, 1170 and 1180. As the channel walls ofthe independent microfluidic channels are impermeable, the fluids withinthe channels flow without mixing.

FIG. 12 depicts a microvascular system 1200, which includes a solidpolymeric matrix 1210, a first microfluidic channel 1220 in the matrix,and a second microfluidic channel 1230 in the matrix. The firstmicrofluidic channel 1220 has a channel wall 1222, and the secondmicrofluidic channel 1230 has a channel wall 1232. The microfluidicchannel walls 1222 and 1232 are impermeable; however, the channel wallscan conduct heat. Thus, the first and second microfluidic channels cantransfer energy, in this instance in the form of heat, between thefluids in the channels.

FIG. 13 depicts a microvascular system 1300, which includes a solidpolymeric matrix 1310, a first microfluidic channel 1320 in the matrix,and a second microfluidic channel 1330 in the matrix. The firstmicrofluidic channel 1320 has a channel wall 1322, and the secondmicrofluidic channel 1330 has a channel wall 1332. The microfluidicchannel walls 1322 and 1332 are permeable, which allows for the transferof mass, in this instance in the form of fluid, between the twomicrofluidic channels.

One example of a microvascular system having permeable channel walls isa water purification system. A microvascular water purification systemincludes a first plurality of microfluidic channels and a secondplurality of microfluidic channels, where the first and second channelsare separated by channel walls containing a nanoporous polymer. A firststream of water having a high concentration of dissolved salts isintroduced into the first plurality of channels and flows through thefirst channels as a pressure is applied. A second stream of water havinga low or negligible concentration of dissolved salts is introduced intothe second plurality of channels. The osmotic pressure differencebetween the two liquids causes water to pass from the second stream intothe first stream, diluting the first stream and reducing theconcentration of dissolved salts in the first stream.

Another example of a microvascular system having permeable channel wallsis an energy production system. An energy production system includes afirst plurality of microfluidic channels and a second plurality ofmicrofluidic channels, where the first and second channels are separatedby channel walls containing an ionic separator. A cathode fluid isintroduced into the first plurality of channels and flows through thefirst channels as a pressure is applied. An anode fluid is introducedinto the second plurality of channels and flows through the secondchannels as a pressure is applied. Ions can pass between the fluidsthrough the ionic separator, creating a flow of charge that can becollected on the opposite sides of the first and second pluralities ofmicrofluidic channels. The collected charge flow can then be convertedinto a conventional electrical current.

Microporous Films

It has also been discovered that microporous films may be made fromcoating precursors that include a thermally degradable polymericmaterial and a thermally stable material having a degradationtemperature higher than the degradation temperature of the thermallydegradable polymeric material. A coating precursor is cast into a filmthat is subject to depolymerization of the thermally degradablepolymeric material, to produce a microporous film. Preferably, themicroporous film has a thickness of from 3 μm to 50 μm, and morepreferably of from 5 μm to 30 μm. Preferred average pore sizes arebetween 50 nm to 10 μm, and more preferred average pore sizes rangebetween 100 nm to 5 μm.

The thermally degradable polymeric material preferably has a degradationtemperature below 280° C., and preferably has a degradation temperatureof at most 250° C. Preferably the thermally degradable polymericmaterial has a degradation temperature between 100 and 250° C.Preferably the thermally degradable polymeric material has a degradationtemperature of at most 220° C., of at most 180° C., of at most 150° C.,or of at most 100° C. The thermally degradable polymeric material mayinclude a poly(hydroxyalkanoate). Examples of poly(hydroxyalkanoate)sinclude poly(3-hydroxybutyrate) (P3HB), poly(4-hydroxybutyrate) (P4HB),poly(3-hydroxy-valerate) (PHV), polycaprolactone, poly(lactic acid)(PLA), poly(glycolic acid) (PGA), and copolymers of the monomeric unitsof these polymers.

Preferably the poly(hydroxyalkanoate) has a weight average molecularweight (Mw) of at least 2,000, and a degradation temperature below 280°C. More preferably, the poly(hydroxyalkanoate) has a degradationtemperature of at most 250° C. Optionally, the depolymerizationtemperature of poly(hydroxyalkanoate)s such as PLA may be reduced byblending the poly(hydroxyalkanoate) with an alkaline earth metal and/ora transition metal. Preferably, the concentration of the metal in thefilm is at least 0.1 percent by weight (wt %). Preferably theconcentration of the metal in the film is at least 0.2 wt %, at least0.5 wt %, at least 1 wt %, at least 2 wt %, at least 2.5 wt %, at least3 wt %, at least 5 wt %, at least 7 wt %, or at least 10 wt %. Theconcentration of the metal in the film may be from 0.1 to 10 wt %, from0.2 to 7 wt %, from 0.5 to 5 wt %, or from 1 to 3 wt %. Preferably themetal is present in the film as MgO, CaO, BaO, SrO, tin(II) acetate,tin(II) oxalate, tin(II) octoate, or scandium triflate (Sc(OTf)₃). Morepreferably the metal is present in the film as strontium oxide, tin(II)oxalate or tin(II) octoate.

When the film is heated to the degradation temperature of the thermallydegradable polymeric material, the thermally degradable polymericmaterial will degrade to form degradants that can be removed, providinga microporous film containing the thermally stable material. Ininstances where the thermally degradable polymeric material has adegradation temperature of at most 250° C., the thermally stablematerial does not thermally degrade at temperatures below 250° C.Preferably, the thermally stable material does not thermally degrade attemperatures below 275° C., at temperatures below 280® C., attemperatures below 300® C., at temperatures below 325® C., or attemperatures below 350® C.

The thermally stable material may include a thermally stable polymericmaterial having a degradation temperature higher than the degradationtemperature of the poly(hydroxyalkanoate), for example a polyamide suchas nylon; a polyester such as poly(ethylene terephthalate) andpolycaprolactone; a polycarbonate; a polyether; an epoxy polymer; anepoxy vinyl ester polymer; a polyimide such as polypyromellitimide (forexample KAPTON®); a phenol-formaldehyde polymer such as bakelite; anamine-formaldehyde polymer such as a melamine polymer; a polysulfone; apoly(acrylonitrile-butadiene-styrene) (ABS); a polyurethane; apolyolefin such as polyethylene, polystyrene, polyacrylonitrile, apolyvinyl, polyvinyl chloride and poly(DCPD); a polyacrylate such aspoly(ethyl acrylate); a poly(alkylacrylate) such as poly(methylmethacrylate); a polysilane such as poly(carborane-silane); and/or apolyphosphazene.

The thermally stable polymeric material may include an elastomer, suchas an elastomeric polymer, an elastomeric copolymer, an elastomericblock copolymer, and/or an elastomeric polymer blend. Examples ofelastomer polymers include polyolefins, polysiloxanes such aspoly(dimethylsiloxane) (PDMS), polychloroprene, and polysulfides;examples of copolymer elastomers may include polyolefin copolymers andfluorocarbon elastomers; examples of block copolymer elastomers mayinclude acrylonitrile block copolymers, polystyrene block copolymers,polyolefin block copolymers, polyester block copolymers, polyamide blockcopolymers, and polyurethane block copolymers; and examples of polymerblend elastomers include mixtures of an elastomer with another polymer.

The thermally stable material may include other ingredients in additionto the thermally stable polymeric material. For example, the thermallystable material may contain one or more particulate fillers,stabilizers, antioxidants, flame retardants, plasticizers, colorants anddyes, fragrances, or adhesion promoters. One type of adhesion promoterthat may be present includes substances that promote adhesion betweenthe thermally stable material and the thermally degradable material, andsubstances that promote adhesion between the thermally stable materialand a polymeric matrix in which the thermally degradable material iscontained.

In instances where the thermally stable material includes a polyimide,the Mw of the polyimide is at least 2,000. Soluble polyimides arepreferred, such as the commercially available MATRIMID® 5218 (Lindberg &Lund A S, Ski, Norway) P84® (Evonik Industries, Essen, Germany), andmixtures thereof. Microporous polyimide films, such as the PI films ofFIG. 20, can be produced by heating the film and depolymerizing thethermally degradable polymeric material. The microporous films have beenfound to be thermally and electrochemically stable, and to also allowthe passage of lithium ions, and may be used in a variety ofapplications, for example as separator in energy storage devices such asbatteries and electrolytic capacitors. Their mechanical strength and lowthermal shrinkage can improve the safety of lithium-ion batteries, wheretheir ability to keep their shape at high temperatures may preventdirect contact between positive electrode materials and negativeelectrode materials.

Methods of making Microporous Films

FIG. 22 illustrates a schematic representation of an exemplary VaSCmethod of making a microporous film. Method 2200 includes providing 2210a coating precursor that includes a thermally degradablepoly(hydroxyalkanoate) and a thermally stable material. Preferredthermally stable materials include thermally stable polymers, forinstance polyimides because of their thermal stability at temperaturetypical of VaSC methods, their mechanical properties, and theirsolubility in solvents traditionally used for solubilizingpolyhydroxyalkanoates. The coating precursor may be a solution formed byco-dissolving or co-dispersing the poly(hydroxyalkanoate) and thethermally stable material in a solvent. Method 2200 further includescasting 2220 a film of the coating precursor, for example by either aspin coating method or doctor blading method. Method 2200 also includessolidifying 2222 the coating precursor by removing at least a portion ofthe solvent from the coating precursor and/or chemical reaction (i.e.curing) of the coating precursor to form a solid film.

Preferably, the coating precursor is solidified at a temperature belowthe degradation temperature of the thermally degradablepoly(hydroxyalkanoate). The solidification of the coating precursor toform a film may include removal of solvent from the precursor and/orchemical reaction (i.e. curing) of the precursor. Examples ofsolidification temperatures include 30° C., 50° C., 75° C., 100° C.,125° C., 150° C. and 180° C. Optionally, a surface treatment may beapplied to the film. Examples of surface treatments includefunctionalization of the film by contacting the film with an oxidizingor reducing atmosphere or by contacting the film with a liquidcontaining a functionalizing reagent. Examples of surface treatmentsinclude applying an adhesion promoter to the film.

Method 2200 further includes heating 2224 the film to a temperature offrom 100 to 250° C., maintaining 2226 the film at a temperature of from100 to 250° C. for a time sufficient to form degradants from thepoly(hydroxyalkanoate), and removing 2228 the degradants from the filmto provide a microporous film. The degradants preferably have an averagemolecular weight less than 500 Daltons. Method 2220 may also includeinterposing 2232 the product microporous film between the positiveelectrode and the negative electrode of an energy-storage device, suchas a lithium ion battery, to provide a separator between the positiveelectrode materials and the negative electrode materials.

The depolymerization temperature of poly(hydroxyalkanoate)s such as PLAmay be reduced by the presence in the film of an alkaline earth metaland/or a transition metal. Preferably the concentration of the metal inthe film is at least 0.2 wt %, at least 0.5 wt %, at least 1 wt %, atleast 2 wt %, at least 2.5 wt %, at least 3 wt %, at least 5 wt %, atleast 7 wt %, or at least 10 wt %. The concentration of the metal in thefilm may be from 0.1 to 10 wt %, from 0.2 to 7 wt %, from 0.5 to 5 wt %,or from 1 to 3 wt %. Preferably the metal is present in the film as MgO,CaO, BaO, SrO, tin(II) acetate, tin(II) oxalate, tin(II) octoate, orscandium triflate (Sc(OTf)3). More preferably the metal is present inthe film as strontium oxide, tin(II) oxalate or tin(II) octoate.

In one example, a metal selected from the group consisting of an alkaliearth metal and a transition metal may be included in the coatingprecursor. In another example, method 2220 may include incorporating2230 the metal in the film through an infusion process. A representativeinfusion process includes contacting the film with a compositionincluding a fluorinated fluid, and a metal selected from the groupconsisting of an alkali earth metal and a transition metal, maintainingthe film and the composition together at a temperature and for a timesufficient to provide a concentration of the metal in the film of atleast 0.1 wt %, and separating the film and the fluorinated fluid. Inone example, films may be infused with a tin(II) oxalate (SnOx) catalystpresent in an aqueous trifluoroethanol (TEE) mixture.

In a set of representative examples, a coating precursor includes amonomer and/or prepolymer that can polymerize to form a polymer, such asa thermally stable polymer as described above with regard to thethermally stable polymeric material. The coating precursor may then besolidified by polymerizing the monomer and/or prepolymer of theprecursor to form the film. Examples of monomers and/or prepolymers thatcan polymerize to form a polymer include cyclic olefins; unsaturatedmonomers such as acrylates, alkylacrylates (including methacrylates andethacrylates), styrenes, isoprene and butadiene; lactones (such ascaprolactone); lactams; epoxy-functionalized monomers, prepolymers orpolymers; functionalized siloxanes; and two-part precursors for polymerssuch as polyethers, polyesters, polycarbonates, polyanhydrides,polyamides, formaldehyde polymers (including phenol-formaldehyde,urea-formaldehyde and melamine-formaldehyde), and polyurethanes.Polymerization of a coating precursor may include crosslinking ofmonomers and/or prepolymers to form an insoluble polymer network.Crosslinking may be performed by a variety of methods, including theaddition of chemical curing agents and/or exposure to radiation such asinfrared radiation (IR; i.e. heat), visible light, or ultravioletradiation (UV).

The following examples are provided to illustrate one or more preferredembodiments of aspects of the present application. Numerous variationscan be made to the following examples that lie within the scope of thepresent application.

EXAMPLES

Materials & Procedures

PLA pellets (Mw=339,000) for forming fibers by solution spinning wereused as received from Purac Biomaterials. Catalyst tin(II) octoate wasobtained from Sigma-Aldrich. Trifluoroethanol (TEE) was obtained fromHalogen Inc. Polyimide resin was obtained from Alfa Aesar. Otherchemicals were all obtained from Sigma-Aldrich unless otherwise noted.

Diglycidyl ether of bisphenol A resin (DGEBA or EPON® 828 was used asreceived from Miller-Stephenson (Danbury, Conn.), and the curing agentEPIKURE® 3300 was used as received from Hexion (Columbus, Ohio). Epoxysamples were prepared using a mass ratio of 22.7 parts per hundred (pph)EPIKURE® 3300 to EPON® 828.

Fiber surface morphology and fiber removal in epoxy matrices were imagedusing a Leica DMR Optical Microscope at various magnifications. Image)software was used to measure fiber diameters from acquired images foreach batch of fibers produced and to measure the fraction of PLA fiberremoved.

Environmental Scanning Electron Microscopy (ESEM, Philips XL3OESEM-FEG)was used to image cross-sections of the holomatrix and to image emptychannels. SEM images were acquired after sputter-coating the samplesurface with carbon or gold-palladium, and were collected usingbackscattered electrons. Selected area elemental analysis was performedby EDS (Energy Dispersive X-ray Spectroscopy, attached to the SEM) witha 20 kV electron source and spot size of 3.0 nm.

An Xradia BioCT (MicroXCT-400) was used to image the apomatrix at 40 keV(8 W power and 200 pA current) at a 4X objective for 5 s exposure times.Rotation intervals were 0.25® for a complete 360® scan. Images werevisualized in 3D with XM3Dviewer and reconstructed in 3D usingXMReconstructor. Reconstructed images were reproduced in Amira toenhance the color and contrast.

Example 1 Formation of Thermally Degradable PLA Fibers

A PLA solution was prepared by dissolving 6 g of PLA pellets indichloromethane at room temperature, and then removing solvent toprovide a solution volume of 35 mL. Catalysts (tin(II) oxalate particlesor tin(II) octoate liquid) were blended into the viscous PLA solution toprovide a 10 wt % tin equivalence to PLA. The mixture was stirred forhalf an hour to disperse the catalyst, resulting in a spinning solution.

A spin chamber was pre-heated to 55° C., and 10 mL of the spinningsolution was transferred to the chamber. The solution was conditioned inthe spin chamber for 5 minutes, and then conditioned outside the chamberfor additional 5 minutes before extrusion, allowing the solution tobecome more concentrated. The spinning solution was then extruded at 55°C. through the chamber at an extrusion speed of 8 cm/hr. The solutionpassed through a spinneret having a diameter from 0.2 mm to 1 mm,forming a single fiber. Two heating chambers below the spinneretprovided additional heat to further evaporate the solvent. The extrudedfiber filament was collected on a Teflon bobbin without applyingadditional stress, and was then air-dried at 50° C. The diameter of thefibers after drying was dependent on the diameter of the spinneret usedin the spinning process. A spinneret diameter of 0.25 mm provided afinal fiber diameter of 0.14±0.02 mm, a spinneret diameter of 0.50 mmprovided a final fiber diameter of 0.42±0.03 mm, and a spinneretdiameter of 1.00 mm provided a final fiber diameter of 0.75±0.05 mm.

SEM analysis of the fibers confirmed a uniform distribution of the SnOcin the spun PLA fiber, which is believed to provide a uniform catalyzeddepolymerization reaction upon heating, resulting in rapid clearing ofthe channel formed from degradation and removal of the fiber core.

The mechanical properties of the spun PLA fibers could be changed bycold-drawing the spun fibers. Cold-drawing fibers may provide anincrease in tensile strength, which is theorized to be due to alignmentof the individual polymer chains within the fiber during the drawingprocess. WAXS analysis of PLA fibers that were cold-drawn after beingspun was consistent with an increase in polymer chain alignment withinthese fibers, as the degree of orientation of pure spun PLA fiber (nocatalyst) increased from 0% when no drawing was performed to 23% whencold-drawing was performed. Spun PLA fibers were drawn to different drawratios, and their failure strengths were studied by a single fibertension test. Cold-drawing appeared to significantly increase the fiberstrength, whereas the presence of SnOc catalyst did not appear to affectfiber failure strength significantly. As the measured fiber failurestrengths were greater than 23 MPa, the fibers were expected to survivethe weaving process without significant failure.

Example 2 Formation of Partially Degradable Fibers

Thermally degradable PLA fibers according to Example 1 were coated witha polysiloxane. A thermally degradable PLA fiber was unwound using atensioner, and then passed through a bath containing a coating precursorcontaining a liquid copolymer of dimethylsiloxane (DMS) and(epoxycyclohexyl)ethylmethylsiloxane (ECMS), and containing the cationicpolymerization initiator (p-isopropylphenyl)(p-methlyphenyl) iodoniumtetrakis(pentafluorophenyl)borate. The cationic initiator can initiatethe reaction of the epoxy groups in the poly(ECMS-co-DMS) when subjectedto UV irradiation.

The fiber was then pulled through a syringe needle tip that had an innerdiameter greater than the diameter of the fiber, providing a uniformlayer of the coating precursor on the exterior surface of the fiber. UVradiation was then applied to the coating precursor on the fiber, curingthe poly(ECMS-co-DMS) into a crosslinked polysiloxane. The resultingcoated fiber was collected on a take-up drum.

The thickness of the coating correlated with the difference between thefiber diameter and the inner diameter of the syringe needle tip throughwhich the fiber was pulled after passing through the bath. Table 1 listsa variety of fiber diameters, syringe needle tip inner diameters, andresulting coated fiber diameters.

TABLE 1 Radial dimensions of PLA fibers, syringe needle tips, and PLAfibers coated with polysiloxane. Diameter of Inner diameter of Diameterof coated PLA fiber (μm) syringe needle tip (μm) PLA fiber (μm) 200 250230 330 270 300 410 350 510 400 500 610 700

The thickness of the coating was relatively uniform along the length ofthe coated fiber. FIG. 14 is a graph of the diameter of a coated fiberas a function of length along the fiber. For a length of 65 centimeters,the diameter of the coated fiber varied from 650 to 750 μm, a variationof 7% from the mean diameter of 709 μm (7.05%=(50 μm/709 μum)×100%).

Example 3 Formation of Composite Containing Partially Degradable Fibers

Coated fibers according to Example 2 were embedded in an EPON® 828:EPIKURE® 3300 epoxy thermoset matrix. The coated fibers were heldstraight in RTV Silicone molds before filling the mold with epoxy. EPON®828 epoxy resin and a cycloaliphatic amine curing agent EPIKURE® 3300were mixed at a ratio of 100:22.7 parts by weight and degassed until airbubbles ceased to form. The post-curing cycle involved heating thespecimens at 82° C. for 90 minutes followed by 150° C. for an additional90 minutes. The cured epoxy thermoset composites were carefully trimmedbefore thermal treatment to expose fiber ends.

FIG. 15 depicts a scanning electron microscopy (SEM) image of thecomposite. The PLA fiber, having a diameter of 500 μm, is in the center,surrounded by the poly(dimethylsiloxane) coating, which in turn issurrounded by the epoxy thermoset matrix.

Example 4 Formation of Microvascular System Containing IndependentMicrofluidic Channels

A composite was formed according to Example 3, where the compositeincluded the epoxy thermoset matrix, and two coated PLA fibers in thematrix. The two coated fibers each included a thermally degradable PLAcore and a polysiloxane coating. The two coated fibers were arrangedperpendicularly and in physical contact at their intersection. After theepoxy was cured, the composite was heated in a sealed vacuum oven(Fisher Isotemp 283) at 200° C. under vacuum (1 torr) for 2 hours,resulting in degradation and removal of the PLA fiber cores.

FIG. 16 depicts a MicroCT image of the microvascular system formed fromthe composite. The system included two microfluidic channels arrangedperpendicularly, and the empty channel within the polysiloxane channelwall was observed in the orthogonal channel.

Example 5

Demonstration of Fluid Non-Communication Between IndependentMicrofluidic Channels in a Microvascular System

A composite was formed according to Example 3, except that the solidpolymer matrix was poly(dimethylsiloxane) (PDMS) formed using thetwo-component siloxane kit Sylgard® 184 (DOW CORNING), which contained amixture of hydroxyl terminated polydimethylsiloxane (HOPDMS) withpolydiethylsiloxane (PDES). The composite included the PDMS matrix andtwo coated PLA fibers in the matrix. The two coated fibers each includeda thermally degradable PLA core and a polysiloxane coating. The twocoated fibers were in physical contact along the length at which thefibers were interwoven. After the PDMS was cured, the composite washeated in a sealed vacuum oven (Fisher Isotemp 283) at 200° C. undervacuum (1 torr) for 2 hours, resulting in degradation and removal of thePLA fiber cores.

FIG. 17 depicts an optical microscopy image of the microvascular systemformed from the composite. The system included two independentmicrofluidic channels having impermeable channel walls. The lack offluid communication between the two microfluidic channels wasdemonstrated by introducing an orange liquid into one channel andintroducing a blue liquid into the other channel. No mixing of thecolors was observed, which indicated that the liquids were not in fluidcommunication despite the physical contact between the microfluidicchannels.

Example 5

Formation of Porous Polyester for Use as a Permeable Coating

A coating precursor was formed from a mixture of PLA and the polyesterpoly(ethylene terephthalate) (PET). A liquid mixture was preparedcontaining 6 weight percent (wt %) PET and 4 wt % PLA in a solvent thatwas a 70:30 mixture of dichloromethane and trifluoroacetic acid. Theliquid mixture was cast into a film by spin-coating at 500 rpm. The filmwas then soaked in dichloromethane to remove the PLA. FIG. 18 depicts anSEM image of the resulting porous film.

Example 6 Formation of Porous Polyimide for Use as a Permeable Coating

A coating precursor was formed from a mixture of PLA and a polyimide.The polyimide was a soluble thermoplastic polyimide having a chemicalname of 1,3-isobenzofurandione, 5,5′-carbonylbis-, polymer with 1-(or3-) (4-aminphenyl)-2,3-dihydro-1,3,3(or1,1,3)-trimethyl-1H-inden-5-amine (CAS #62929-02-6). A solution wasprepared containing from 1-2 wt % polyimide and from 1-1.5 wt % PLA inchloroform. The solution was cast into a film either by spin-coating orby contacting a layer of the solution with a doctor blade. The resultingpolymer blend film had a thickness of from 5 to 30 μm. FIG. 19 depictsSEM images of the polymer blend film formed using a doctor blade. Themicrospheres are believed to be particles containing the polyimide. Thepolymer blend films were heated in a sealed vacuum oven (Fisher Isotemp283) at 200° C. under vacuum (1 torr) for 2 hours, resulting indegradation and removal of the PLA phase. FIG. 20 depicts SEM images ofthe resulting porous film. The microspheres are believed to be particlescontaining the polyimide that have sintered together to form a cohesivefilm. The pores left by the removal of the PLA phase had widths ofapproximately 1 μm.

In another example, a coating precursor was formed from a mixture of PLAand a polyimide in a binary solvent. A solution was prepared containingfrom 1-2 wt % polyimide and from 1-1.5 wt % PLA in a solvent that was a10:1 mixture of chloroform and dioxane. As described above, the solutionwas cast into a film and heated in a sealed vacuum oven (Fisher Isotemp283) at 200° C. under vacuum (1 torr) for 2 hours, resulting indegradation and removal of the PLA phase. FIG. 21 depicts SEM images ofthe resulting porous film. As the PLA and polyimide formed abicontinuous film when cast from the binary solvent, the resultingporous film had a continuous polyimide phase.

Example 7 Microporous Polyimide Battery Separators

The coating precursor of Example 6 was formed as solution having aPI:PLA ratio 40:60 (wt/wt) in chloroform. The PI was a1,3-isobenzofurandione having a molecular weight of about 38 kDa, andthe PLA of about 55 KDa. The solution was cast into a film either byspin coating or by contacting a film of the solution with a doctorblade, and the film was left to dry for 10 minutes. The resultingpolymer blend film was heated in a sealed vacuum oven to a temperatureof 280° C., removing the PLA and resulting in microporous polyimidefilms having a thickness of 5 to 30 μm.

Button lithium-ion cells were prepared with a mesocarbon microbeads(MCMB) graphite powder (Enerland, Korea) as anode material,Li(Ni_(1/3)Co_(1/3)Mn_(1/3))O₂ (NMC) as cathode material, and LiPF₆ inethylene carbonate (EC) as solvent. Cathodes and anodes were cut to theappropriate size using a 1.59 punch. A first set of batteries featuredmicroporous polyimide separators having a thickness of 20 μm, and asecond set of control batteries were fitted with CELGARD® 2325 (Celgard,Charlotte, N.C.), a type of traditional, commercially available trilayer(PP/PE/PP) separator membranes. The cells with the microporous polyimideseparator exhibited impedance and discharge capacity values very closeto those of coin cells featuring the Celgard® 2325. This is illustratedin the linear sweep voltammograms (LSV) of FIG. 23, which were taken onboth types of cells (scan rate: 1.0 mV/s).

FIG. 24A illustrates the charge and discharge curves of a coin cellfeaturing the microporous polyimide separator, exhibiting a specificcapacity of about 193.91 mAh/g with a capacity retention ratio of 96.3%,nearly the same as that of a coin cell assembled with a commercialCELGARD® 2325 separator (192.76 mAh/g, 95.1%). FIG. 24B illustrates thecycling behavior of coin cells featuring the microporous polyimideseparator, as measured at a 0.1 C rate at room temperature; a stablecycling behavior was observed and no abnormal or unstablecharge-discharge profiles were observed. Coin cells featuringmicroporous polyimide separators prepared from coating precursors havingPI:PLA ratios of 50:50 and 60:40, respectively, exhibited a similarbehavior. FIG. 24C illustrates the charge and discharge capacities ofbatteries featuring the polyimide separators as a function of thedischarge current density (i.e., discharge C-rate). Noticeably, thedischarge capacities dropped at higher discharge current densities,where the influence of ionic transport on the ohmic polarization (IRdrop) is more significant. FIG. 24C shows that the polyimide separatorsallowed for successful battery cycling at different charge densities.

The thermal shrinkage of the microporous polyimide separators wasassessed by changes in size (as measured by length and width) followingheat treatment at various temperatures for 30 minutes. FIG. 25Aillustrates how the polyimide separator hardly changed over a widetemperature range, whereas conventional, commercially available CELGARD®2325 separators exhibited amounts of shrinkage that were directlyproportional to the heat treatment temperature. This difference inthermal shrinkage between the two types of separators increased as thetemperature of the heat treatment approached 140° C. FIG. 25B is aphotograph of the microporous polyimide separator and of the CELGARD®2325 separator prior to and following heat treatment at a temperature of140° C. for 30 minutes. The CELGARD® 2325 separator underwent a thermalshrinkage of 32%. In contrast, the microporous polyimide separatorshowed little change due to thermal shrinkage. The microporous polyimideseparator was thermally stable. Without wishing to be bound to anyparticular theory, this stability is believed to be a result of itsmelting temperature of more than 480° C. Therefore, it appears that thesuperior thermal stability of the polyimide prevents thermal shrinkage,thereby providing a safe, thermally stable battery separator.

Example 8 Formation of Sacrificial Fibers Coated with a PI/PLA Blend

A coating precursor was formed from a mixture of a soluble polyimide(Matrimid® 5218, number average molecular weight (Mn) 38,000; Mw 73,000)and PLA having a Mn of 55,400 (Natureworks, Minnetonka, Minn.). Asolution was prepared containing 2-2.5 wt % polyimide, 1-1.5 wt % PLA,and 0.2-0.4 wt % tin(II) octoate in chloroform. Sacrificial fibers weredip-coated with the solution, leaving a coating containing a mixture ofphase-separated polyimide and PLA. FIG. 26 shows an optical micrographof the cross-section of a coated fiber.

In another example, sets of two PLA fibers held at a fixed proximitywere dip-coated into a solution containing 2-2.5 wt %, 1-1.5 wt % PLA,and 0.2-0.4 wt % tin(II) octoate in chloroform. FIG. 27 shows an opticalmicrograph of the cross-section of the collectively coated fibers.

Definitions

To provide a clear and more consistent understanding of thespecification and claims of this application, the following definitionsare provided.

The term “polymeric” means a substance that includes a polymer.

The term “polymer” means a substance containing more than 100 repeatunits. The term “polymer” includes soluble and/or fusible moleculeshaving long chains of repeat units, and also includes insoluble andinfusible networks. The term “prepolymer” means a substance containingless than 100 repeat units and that can undergo further reaction to forma polymer. Unless otherwise specified, polymer molecular weights aregiven in Daltons.

The term “matrix” means a continuous phase in a material.

The term “matrix precursor” means a composition that will form a polymermatrix when it is solidified. A matrix precursor may include a monomerand/or prepolymer that can polymerize to form a solid polymer matrix. Amatrix precursor may include a polymer that is dissolved or dispersed ina solvent, and that can form a solid polymer matrix when the solvent isremoved. A matrix precursor may include a polymer at a temperature aboveits melt temperature, and that can form a solid polymer matrix whencooled to a temperature below its melt temperature.

The term “woven structure” means a single ply of an assembly of threads,where the threads are oriented in at least 2 directions within the ply.

The term “microfluidic channel” means a substantially tubular structurehaving a diameter less than 1,000 micrometers.

The term “microfluidic network” means a plurality of channels having aplurality of interconnects, where at least a portion the channels have adimension less than 1,000 micrometers.

The term “fluid communication” means that two objects are in anorientation, and within a sufficient proximity to each other, such thatfluid can flow from one object to the other. The term “fluid” means asubstance in the liquid or gaseous state. In one example, if amicrofluidic channel embedded in a matrix is in fluid communication witha surface of the matrix, then fluid can flow from the channel onto thesurface.

While various embodiments of the present application have beendescribed, it will be apparent to those of ordinary skill in the artthat other embodiments and implementations are possible within the scopeof the present application. Accordingly, the present application is notto be restricted except in light of the attached claims and theirequivalents.

What is claimed is:
 1. A partially degradable polymeric fiber comprisinga thermally degradable polymeric core and a thermally stable coatingsurrounding substantially all of the thermally degradable polymericcore, where the thermally stable coating comprises a thermally stablepolymer having a higher degradation temperature than the degradationtemperature of the thermally degradable polymeric core, where thethermally degradable polymeric core comprises a polymeric matrixcomprising a poly(hydroxyalkanoate) and 0.1-10 wt % of a metal selectedfrom the group consisting of an alkaline earth metal and a transitionmetal, and where the thermally stable coating forms a wall around amicrofluidic channel when the thermally degradable polymeric core isdegraded and the degradants are removed.
 2. The fiber of claim 1, wherethe degradation temperature of the thermally degradable polymeric coreis at most 250° C., and the degradation temperature of the thermallystable polymer of the coating is at least 275° C.
 3. The fiber of claim2, where the degradation temperature of the thermally degradablepolymeric core is at most 220° C.
 4. The fiber of claim 3, where thedegradation temperature of the thermally degradable polymeric core is atmost 180° C.
 5. The fiber of claim 1, where the thermally stable polymerof the coating comprises a polysiloxane, polychloroprene, polysulfide,poly(hydroxyalkanoate), polyimide, polyester, polycarbonate, polyether,epoxy polymer, epoxy vinyl ester polymer, polyamide, phenol-formaldehydepolymer, amine-formaldehyde polymer, polysulfone,polyacrylonitrile-butadiene-styrene, polyurethane, polyolefin,polystyrene, polyacrylonitrile, polyvinyl chloride, poly(DCPD),polyacrylate, poly(alkylacrylate), polysilane, polyphospazene, or acombination thereof.
 6. The fiber of claim 5, where the thermally stablecoating comprises: (i) a polysiloxane; (ii) a polyimide; or (iii) apoly(hydroxyalkanoate) in combination with a polyimide or a polyester,where the poly(hydroxyalkanoate) of the thermally stable coating isdifferent than or the same as the poly(hydroxyalkanoate) of thepolymeric matrix of the thermally degradable polymeric core.
 7. Thefiber of claim 1, where the poly(hydroxyalkanoate) of the polymericmatrix comprises poly(lactic acid), poly(3-hydroxybutyrate),poly(4-hydroxybutyrate), poly(3-hydroxy-valerate), polycaprolactone,poly(glycolic acid) and/or a copolymer of monomeric units of thepreceding polymers.
 8. The fiber of claim 7, where thepoly(hydroxyalkanoate) of the polymeric matrix of the thermallydegradable polymeric core comprises poly(lactic acid).
 9. The fiber ofclaim 1, where the metal comprises MgO, CaO, BaO, SrO, tin(II) acetate,tin(II) oxalate, tin(II) octoate, or scandium triflate (Sc(OTf)₃). 10.The fiber of claim 1, where the thermally stable coating furthercomprises a blend of the thermally stable polymer and a thermallyunstable polymer having a degradation temperature that is the same as orless than the degradation temperature of the thermally degradablepolymeric core.
 11. A method of making a partially degradable polymericfiber that comprises a thermally degradable polymeric core and athermally stable coating surrounding substantially all of the thermallydegradable polymeric core, where the thermally stable coating comprisesa thermally stable polymer having a higher degradation temperature thanthe degradation temperature of the thermally degradable polymeric core,where the thermally degradable polymeric core comprises a polymericmatrix comprising a poly(hydroxyalkanoate) and 0.1-10 wt % of a metalselected from the group consisting of an alkaline earth metal and atransition metal, and where the thermally stable coating forms a wallaround a microfluidic channel when the thermally degradable polymericcore is degraded and the degradants are removed, the method comprising:(i) combining an initial fiber comprising the poly(hydroxyalkanoate) ofthe polymeric matrix, and a composition comprising a fluorinated fluidand the metal; (ii) maintaining the initial fiber and the compositiontogether at a temperature and for a time sufficient to provide a metalconcentration of 0.1-10 wt % in the initial fiber and form the thermallydegradable polymeric core; (iii) separating the initial fiber from thefluorinated fluid; and (iv) depositing the thermally stable coating onsubstantially all of the surface of the initial fiber to provide thepartially degradable polymeric fiber.
 12. The method of claim 11,further comprising (v) applying a surface treatment to the thermallystable coating.
 13. The method of claim 11, where thepoly(hydroxyalkanoate) of the polymeric matrix of the thermallydegradable polymeric core comprises poly(lactic acid).
 14. The method ofclaim 11, where the metal comprises MgO, CaO, BaO, SrO, tin(II) acetate,tin(II) oxalate, tin(II) octoate, or scandium triflate (Sc(OTf)₃). 15.The method of claim 11, where the depositing of step (iv) comprisescontacting substantially all of the surface of the initial fiber with aliquified coating precursor, and solidifying the coating precursor toprovide the thermally stable coating surrounding substantially all ofthe thermally degradable polymeric core.
 16. A method of making apartially degradable polymeric fiber that comprises a thermallydegradable polymeric core and a thermally stable coating surroundingsubstantially all of the thermally degradable polymeric core, where thethermally stable coating comprises a thermally stable polymer having ahigher degradation temperature than the degradation temperature of thethermally degradable polymeric core, where the thermally degradablepolymeric core comprises a polymeric matrix comprising apoly(hydroxyalkanoate) and 0.1-10 wt % of a metal selected from thegroup consisting of an alkaline earth metal and a transition metal, andwhere the thermally stable coating forms a wall around a microfluidicchannel when the thermally degradable polymeric core is degraded and thedegradants are removed, the method comprising: (i) forming a spinningliquid comprising the poly(hydroxyalkanoate) of the polymeric matrix, asolvent, and the metal; (ii) passing the spinning liquid through aspinneret to form a wet fiber comprising the poly(hydroxyalkanoate) ofthe polymeric matrix and the metal; (iii) drying the wet fiber toprovide a metal concentration of 0.1-10 wt % in the dried fiber and formthe thermally degradable polymeric core; and (iv) depositing thethermally stable polymeric coating on substantially all of the surfaceof the dried fiber to provide the partially degradable polymeric fiber.17. The method of claim 16, further comprising cold-drawing the driedfiber between steps (iii) and (iv).
 18. The method of claim 16, wherethe poly(hydroxyalkanoate) of the polymeric matrix of the thermallydegradable polymeric core comprises poly(lactic acid).
 19. The method ofclaim 16, where the metal comprises MgO, CaO, BaO, SrO, tin(II) acetate,tin(II) oxalate, tin(II) octoate, or scandium triflate (Sc(OTf)₃). 20.The method of claim 16, where the depositing of step (iv) comprisescontacting substantially all of the surface of the dried fiber with aliquified coating precursor , and solidifying the coating precursor toprovide the thermally stable coating surrounding substantially all ofthe thermally degradable polymeric core.